Embodiments of the present invention relate to analog circuits, and more particularly, to operational amplifiers.
Many prior art CMOS (Complementary-Metal-Oxide-Semiconductor) operational amplifiers rely upon external biasing in order to bias in the saturation region various FETs (Field-Effect-Transistor) that serve as current sources (or active loads) in the operational amplifiers. However, external biasing may be sensitive to process technology, supply voltage, and temperature. Furthermore, because the overall gain and output resistance of an operational amplifier may both be very high and difficult to accurately model, the output node voltages for zero differential input voltage is very difficult to predict. In general, these node voltages should be at or near Vcc/2 for zero differential input voltage, where Vcc is the supply voltage.
Other prior art operational amplifiers have utilized various methods of self-biasing with negative feedback, so that the output node voltages are nominally at Vcc/2. However, for some of these prior art operational amplifiers, external biasing is not completely eliminated, and for others, some or all the FETs that serve as the current sources are biased in their linear region instead of their saturation region, resulting in reduced voltage gain. The present invention addresses these problems.